One of the topics of paramount importance in the development of Cluster and Grid middleware is the impact of faults since their occurrence probability in a Grid infrastructure and in large-scale distributed system is actually very high. MPI (Message Passing Interface) is a popular abstraction for programming distributed computation applications. FAIL is an abstract language for fault occurrence description capable of expressing complex and realistic fault scenarios. In this paper, we investigate the possibility of using FAIL to inject faults in a fault-tolerant MPI implementation. Our middleware, FAIL-MPI, is used to carry quantitative and qualitative faults and stress testing.